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The present invention relates to mounting techniques for semiconductor devices and more specifically to a method and system for providing different loading forces for a semiconductor device and an associated heat sink.
Semiconductor devices are often mounted to a printed circuit board via a semiconductor device socket. By mounting a semiconductor device in this manner, the device may be readily removed and replaced in the event of a device failure.
Semiconductor device packages take a number of forms. In a lidded semiconductor device, a semiconductor die is mounted to a substrate. A heat spreader plate having a cavity on the underside of the plate that is sized to receive the die is mounted to the substrate with the die positioned within the cavity. Thermal epoxy is underfilled in the cavity surrounding the die. This structure provides mechanical rigidity for the semiconductor device and allows for heat removal from the die. More specifically, heat removal from the semiconductor die may be accomplished by abutting a heat sink to the top surface of the heat spreader plate. Heat is conveyed from the die to the thermal epoxy and from the thermal epoxy to the heat spreader plate and the abutting heat sink. To obtain efficient heat removal, the heat sink must apply sufficient force to the top surface of the lidded device to achieve good thermal conductivity.
The above-described technique achieves less that optimal heat removal since thermal epoxy is not an ideal conductor of heat. The failure to adequately remove heat from a semiconductor device can result in the failure of the device.
Recognizing that thermal epoxy does not provide a highly efficient medium for heat removal from the semiconductor die, in some systems, lidless semiconductor devices are employed and a heat sink is placed in abutting relation with the die to achieve better heat conductivity to the heat sink. Lidless semiconductor devices that have a ball grid array have been soldered directly to a circuit board. A heat sink has been mounted above such devices and pressure has been applied to the heat sink to urge the heat sink against the top surface of the die so as to provide an effective thermally conductive interface between the top surface of the die and the heat sink.
While it is desirable to employ sockets for the mounting of semiconductor devices, the mounting of a lidless semiconductor device such as a land grid array (LGA) device is problematic. A minimum pressure is required to assure proper electrical conductivity between the contacts on the underside on an LGA device and associated conductive contacts which may comprise elastomeric columnar contacts within an Metalized Particle Interconnect (MPI) socket. Considerably less pressure is required to provide proper thermal conductivity between a heat sink and the top surface of a lidless device. The application of forces to the top surface of the semiconductor device that are sufficient to obtain good electrical conductivity between the semiconductor device contacts and the socket contacts may result in damage to the semiconductor die.
Accordingly it would be desirable to be able to mount a lidless or lidded semiconductor device within a socket, such as an LGA socket in a manner that provides the forces needed to assure proper electrical conductivity at the respective conductive contact interfaces while not subjecting the semi-conductor die to potential damage as a consequence of expressive forces imported by a heat sink.
In accordance with the present invention, a mounting system for a semiconductor device is disclosed that maintains a first predetermined compressive force on the contacts of a semiconductor socket, such as an MPI socket, and which allows for the application of a second lesser predetermined force to be applied by a heat sink surface to a semiconductor device so as to obtain desired heat removal from the device during use. The first predetermined compressive force is obtained via a novel load cell. The load cell includes a backer plate having shouldered standoffs mounted in respective corners of the backer plate, a load distribution plate having openings in the respective corners that align with the shouldered standoffs to allow passage of the shouldered standoffs therethrough, and a bow spring comprising a 3-dimensional spring metal structure having flanges extending outward from a raised center portion. The bow spring is disposed between the backer plate and the load distribution plate and the backer plate is captively affixed to the load distribution plate using flanges that wrap around the lower surface of the backer plate and pre-load the load cell.
The shouldered standoffs extend through holes in an insulator and a printed circuit board. A semiconductor device socket, such as an MPI socket, is precisely positioned on the opposing side of the printed circuit board from the load cell such that the socket contacts mate with corresponding conductive contacts on the circuit board. A semiconductor device is disposed within the socket and a pressure plate is disposed in abutting relation with the exterior periphery of the top surface of the semiconductor device. The shouldered standoffs extend through holes provided in the pressure plate. The semiconductor device and the semiconductor device socket are compressively loaded ;by drawing the pressure plate toward the load cell via nuts applied to screw threads on the shouldered standoffs, via cams, or via any suitable lever mechanism.
A heat sink having a base has a pedestal extending from the base that contacts the semiconductor die or a lid on a lidded semiconductor. The pedestal is urged into abutting contact with the semiconductor via springs that engage the pressure plate and control the force applied to the semiconductor die or lid.
Other features, aspects and advantages of the above-described load cell and integrated circuit mounting system will be apparent from the Detailed Description of the Invention that follows.